Many olefin polymerization catalysts are known, including conventional Ziegler-Natta catalysts. While these catalysts are inexpensive, they exhibit low activity, produce polymers having narrow to medium molecular weight distributions (M.sub.w /M.sub.n &gt;4), and are generally poor at incorporating a-olefin comonomers. To improve polymer properties, highly active single-site catalysts, in particular metallocenes, are beginning to replace Ziegler-Natta catalysts. Although more expensive, the new catalysts give polymers with narrow molecular weight distributions, and good comonomer incorporation, which allows easier production of low-density polymers. One disadvantage of metallocene catalysts is that they tend to produce lower molecular weight polymers at higher temperatures.
Recent attention has focused on developing improved single-site catalysts in which a cyclopentadienyl ring ligand is replaced by a heteroatomic ring ligand. These catalysts may be referred to generally as "heterometallocenes."
In particular, U.S. Pat. No. 5,554,775 discloses single-site catalysts containing a boraaryl moiety such as boranaphthalene or boraphenanthrene. U.S. Pat. No. 5,539,124 discloses catalysts containing a pyrrolyl ring, i.e., an "azametallocene." Further, U.S. Pat. No. 5,637,660 discloses catalysts in which a cyclopentadienyl moiety of a metallocene is replaced by a readily available quinolinyl or pyridinyl ligand. In addition, PCT Int. Appl. WO 96/34021 discloses azaborolinyl heterometallocenes wherein at least one aromatic ring includes both a boron atom and a nitrogen atom.
Single-site catalysts are typically soluble in the polymerization reaction medium and are therefore valuable for solution processes. However, for gas-phase, slurry, and bulk monomer processes, it is useful to immobilize the catalyst on a carrier or support in order to control polymer morphology. Much effort has focussed on supporting metallocene and Ziegler-Natta catalysts. Various supports are taught, particularly inorganic oxides. Support modification techniques, which can improve activity, are also known. For example, supports for Ziegler-Natta catalysts modified with organomagnesiums, organosilanes, and organoboranes are disclosed in U.S. Pat. Nos. 4,508,843, 4,530,913, and 4,565,795. Metallocene catalyst support modification with organosilanes and aluminum, zinc, or silicon compounds is taught in U.S. Pat. Nos. 4,808,561 and 5,801,113.
In contrast, relatively little is known about supporting heterometallocenes. U.S. Pat. No. 5,744,417 discloses a silylamine polymer support, but the examples use only a metallocene catalyst. U.S. Pat. Nos. 5,554,775, 5,539,124, and 5,637,660 and PCT Int. Appl. WO 96/34021 teach that heterometallocenes can be supported on inorganic oxides, but these references give no examples.
Many heterometallocenes are inherently unstable. U.S. Pat. Nos. 5,554,775 and 5,539,124 teach that the catalyst should be used shortly after preparation because activity is lost on storage. Moreover, our own initial efforts to make supported heterometallocenes using untreated supports were largely unsuccessful (see Comparative Example 8 and Table 6 below). New supports for heterometallocenes would ideally provide for increased storage stability.
In sum, new supported heterometallocene catalysts and methods of making them are needed. Particularly valuable supported catalysts would have improved shelf-life and would give polymers with enhanced properties. Ideally, the new supports would have a negligible negative effect on catalyst activity.